1. Field of the Invention
The present invention relates to an integrated semiconductor laser array apparatus provided with an integrated semiconductor laser array and a circuit for driving the array.
2. Description of the Related Art
With the progress in the optical communications technologies, studies are being made on applications of optical transmission technologies to high-speed data transmission between communications facilities not only in trunk line systems but also of transmission terminals and exchanges, as well as between computers or units in the computer. In such optical transmission technologies, the optical parallel transmission whereby a number of signals are transmitted in parallel is an effective method. As compared with the electric parallel transmission system, the optical parallel transmission system has excellent features in terms of transmission speed, transmission distance, electromagnetic induction noise, etc., and therefore researches on the development of the system are being advanced in various fields.
With the increase in the capacity of the optical transmission system, the need for the optical parallel transmission system capable of processing a number of channels, as seen in the optical cross connect system, has become great. As the light source on the transmitter side in the optical parallel transmission system, an integrated array of light emitting elements, which is constructed of a plurality of light emitting elements formed integral, is used for converting a number of electric signals en bloc into optical signals. While there are light emitting diodes (LED) and semiconductor lasers, or laser diodes, as practical light emitting elements, it is preferable to use the semiconductor lasers in view of their high transmission speed, adaptability to wavelength multiplexing, etc.
With the semiconductor laser in general, such characteristics as the threshold current for oscillation and light emitting efficiency (differential quantum efficiency) greatly change depending on the temperature more than with the light emitting diode, and therefore a stabilized operation of the semiconductor laser is desired under varying temperature conditions. As a technique to stabilize the output of a single semiconductor laser, there is a method hitherto known in which a portion of the optical output of the semiconductor laser is sensed by a photodiode and the bias current for the semiconductor laser is controlled by an automatic power control circuit (APC) such that the level of the sensed light is kept constant.
However, if it is attempted to apply the above described technique to an integrated semiconductor laser array to thereby control the output of each individual semiconductor laser element, a problem arises that the circuit necessary for the control becomes large in scale depending on the number of the elements and, hence, the structure of the apparatus becomes complex.